1. Field of the Invention
The present invention relates to a toroidal type continuously variable transmission and, for example, to a single-cavity, toroidal type continuously variable transmission used as an automotive transmission.
2. Description of Related Art
Research is being conducted on using a toroidal type continuously variable transmission schematically shown in FIGS. 1 and 2 as an automotive transmission. This toroidal type continuously variable transmission has an input disc 2, namely a first disc, concentrically supported with an input shaft 1, and an output disc 4, namely a second disc, fixed on an end of an output shaft 3 concentrically disposed with the input shaft 1 as disclosed, for example, in Japanese Utility Model Application Laid-open No. 62-71465. Provided inside a casing which houses the toroidal type continuously variable transmission are trunnions 6, 6 which rock about pivots 5, 5 torsionally positioned in relation to the input shaft 1 and the output shaft 3.
The trunnions 6, 6 are respectively provided with the pivots 5, 5 on the outer surfaces at both ends thereof. At the centers of the trunnions 6, 6, the proximal ends of displacement shafts 7, 7 are supported, and the tilt angle of the displacement shafts 7, 7 can be adjusted by rocking the trunnions 6, 6 about the pivots 5, 5. Power rollers 8, 8 are rotatably supported around the displacement shafts 7, 7 supported by the trunnions 6, 6, the power rollers 8, 8 being held between the input disc 2 and the output disc 4.
The sections of inner surfaces 2a and 4a of the input disc 2 and the output disc 4, respectively, which are opposed to each other, are shaped like recessed surfaces obtained by turning arcs, which have the pivots 5 as the centers thereof, about the input shaft 1 and the output shaft 3. The peripheral surfaces 8a, 8a of the power rollers 8, 8 formed into spherical convex surfaces are held against the inner surfaces 2a and 4a.
A loading-cam type pressuring device 101 is provided between the input shaft 1 and the input disc 2; the pressuring device 101 resiliently presses the input disc 2 toward the output disc 4. The pressuring device 101 is constructed by a cam disc 102 which rotates together with the input shaft 1, and a plurality of (e.g. four) rollers 12, 12 held by a retainer 11. Formed on one surface of the cam disc 102, namely the left surface in FIGS. 1 and 2, is a cam surface 13 which is an irregular surface formed in the circumferential direction; a similar cam surface 14 is formed also on the outer surface of the input disc 2, namely the right surface in FIGS. 1 and 2. The plurality of rollers 12, 12 are rotatably supported about a radial axis in relation to the center of the input shaft 1.
At operating the toroidal type continuously variable transmission constructed as set forth above, when the cam disc 102 rotates as the input shaft 1 rotates, a cam surface 13 presses the plurality of rollers 12, 12 against a cam surface 14, which is an outer surface of the input disc 2. This causes the input disc 2 to be pressed against the power rollers 8, 8 and rotated due to the pair of the cam surfaces 13 and 14 pressing against the plurality of rollers 12, 12. The rotation of the input disc 2 is transmitted to the output disc 4 via the power rollers 8, 8, causing the output disc 4 to rotate the output shaft 3 secured to the output disc 4.
When changing the rotational speed ratio (speed change ratio) of the input shaft 1 to the output shaft 3, in order to reduce the speed between the input shaft 1 and the output shaft 3, the trunnions 6, 6 are rocked about the pivots 5, 5 to tilt the displacement shafts 7, 7 so that the peripheral surfaces 8a, 8a of the power rollers 8, 8 respectively come in contact with a closer-to-the-center portion of the inner surface 2a of the input disc 2 and a closer-to-the-outer-periphery portion of the inner surface 4a of the output disc 4 as shown in FIG. 1.
To increase the speed, the trunnions 6, 6 are rocked about the pivots 5, 5 to tilt the displacement shafts 7, 7 so that the peripheral surfaces 8a, 8a of the power rollers 8, 8 respectively come in contact with a closer-to-the-outer-periphery portion of the inner surface 2a of the input disc 2 and a closer-to-the-center portion of the inner surface 4a of the output disc 4 as shown in FIG. 2. Setting the tilt angles of the displacement shafts 7, 7 at the midpoints illustrated in FIGS. 1 and 2, respectively, enables an intermediate speed change ratio to be obtained between the input shaft 1 and the output shaft 3.
FIGS. 3 and 4 illustrate a toroidal type continuously variable transmission which has been disclosed in the microfilm of Japanese Utility Model Application Laid-open No. 1-173552 and which has been further embodied. The input disc 2 and the output disc 4 are rotatably supported around an input shaft 15, which is a rotating shaft shaped like a circular pipe, via needle bearings 16, 16, respectively. The cam disc 102 is spline-engaged on the outer peripheral surface of the left end of the input shaft 15 shown in FIG. 3. A jaw 17 prevents the cam disc 102 from moving away from the input disc 2. In the loading-cam type pressuring device 101, the cam disc 102 and the rollers 12, 12 press the input disc 2 toward the output disc 4 as the input shaft 15 rotates so as to rotate it. An output gear 18 is joined to the output disc 4 through keys 19, 19 so that the output disc 4 and the output gear 18 rotate in synchronization.
The pivots 5, 5 provided on both ends of a pair of the trunnions 6, 6 are supported by a pair of support plates 20, 20 such that they are free to be displaced in the rocking direction and the axial direction, i.e. in the front-back longitudinal direction in FIG. 3 and the left-right lateral direction in FIG. 4, respectively. The displacement shafts 7, 7 are supported in round holes 23, 23, respectively, which are formed at the midpoints of the trunnions 6, 6. The displacement shafts 7, 7 respectively have support shafts 21, 21 and pivot shafts 22, 22 which are parallel to each other and eccentric. The support shafts 21, 21 are rotatably supported in the round holes 23, 23 via radial needle bearings 24, 24. The power rollers 8, 8 are rotatably supported around the pivot shafts 22, 22 via radial ball-and-roller bearings such as radial needle bearings 25, 25.
The pair of displacement shafts 7, 7 are provided such that they are 180 degrees opposite from each other about the input shaft 15. The pivot shafts 22, 22 of the displacement shafts 7, 7 are decentered from the support shafts 21, 21 in the same direction with respect to the rotational direction of the input disc 2 and the output disc 4, i.e. in the opposite lateral direction in FIG. 4; the eccentric direction is nearly orthogonal to the direction in which the input shaft 15 is disposed (in the lateral direction in FIG. 3 or in the longitudinal direction in FIG. 4). Hence, the power rollers 8, 8 are supported so that they are slightly free to shift in the direction in which the input shaft 15 is oriented. As a result, even if the power rollers 8, 8 have come to be apt to shift in the axial direction of the input shaft 15 (in the lateral direction in FIG. 3 or in the longitudinal direction in FIG. 4) due to the dimensional errors of the component parts, the elastic deformation taking place when power is transmitted, or for other reason, such a shift can be absorbed without causing an undue force to be applied to the component parts.
Provided between the outer surfaces of the power rollers 8, 8 and the middle inner surfaces of the trunnions 6, 6 are thrust ball-and-roller bearings such as thrust ball bearings 26, 26, and thrust bearings such as thrust needle bearings 27, 27 which support the thrust load applied to outer rings 30, 30 which will be discussed below in the order in which they are listed from the outer surfaces of the power rollers 8, 8. The thrust ball bearings 26, 26 allow the power rollers 8, 8 to rotate while supporting, at the same time, the load in the thrusting direction applied to the power rollers 8, 8. The thrust ball bearings 26, 26 are respectively constructed by a plurality of balls 29, 29, annular holders 28, 28 which rollably hold the balls 29, 29, and the outer rings 30, 30 serving as thrust track rings. The inner ring tracks or inner race tracks of the thrust ball bearings 26, 26 are formed on the outer surfaces of the power rollers 8, 8, while the outer ring tracks or outer race tracks thereof are formed on the inner surfaces of the outer rings 30, 30.
The thrust needle bearings 27, 27 are constituted by a race 31, a holder 32 and needles 33, 33; the race 31 and the holder 32 are combined such that they are free to slightly shift in the rotational direction. These thrust needle bearings 27, 27 are held between the inner surfaces of the trunnions 6, 6 and the outer surfaces of the outer rings 30, 30 with the races 31, 31 held against the inner surfaces of the trunnions 6, 6. Such thrust needle bearings 27, 27 allow the pivot shafts 22, 22 and the outer rings or outer races 30, 30 to rock around the support shafts 21, 21 while at the same time supporting the thrust load applied to the outer rings 30, 30 by the power rollers 8, 8.
Drive rods 34, 34 are joined to one end, namely the left end in FIG. 4, of the trunnions 6, 6 and drive pistons 35, 35 are secured to the middle outer peripheral surface s of the drive rods 34, 34. These drive pistons 35, 35 are oiltightly fitted in drive cylinders 36, 36, respectively.
A first ball-and-roller bearing 39 is installed at a securing portion between a support wall 38 provided in a casing 37 and the input shaft 15; and a second ball-and-roller bearing 40 is installed at a securing portion between the support wall 38 and the output gear 18. In the example illustrated, as the ball-and-roller bearings 39 and 40, angular ball bearings are used by combining them with their back surfaces facing each other, the directions of the contact angles thereof being opposite from each other. More specifically, outer rings 41, 41 making up the ball-and -roller bearings 39 and 40 are internally fitted in a round hole 43 formed in the support wall 38 and the end surfaces of the outer rings 41, 41 are butt-joined via a spacer 42.
Of inner rings or inner races 44, 44 making up the ball-and-roller bearings 39 and 40, the one constituting the first ball-and-roller bearing 39 is fitted to the outside of a holder 45 which is externally fitted in the outer peripheral surface of the input shaft 15 such that it may be displaced in the axial direction. A flathead spring 47 is clamped between the rear surface (the right surface in FIG. 3) of the holder 45 and a loading nut 46 secured to the outer peripheral surface of the input shaft 15. The flathead spring 47 is provided to apply pre-pressure so as to resiliently hold the inner surfaces 2a and 4a against the peripheral surfaces 8a, 8a of the power rollers 8, 8 even while the pressuring device 101 is "OFF". The inner ring 44 constituting the second ball-and-roller bearing 40 is fitted and secured to the outside of a support cylindrical section 48 formed on the inner peripheral edge portion of the output gear 18.
In the toroidal type continuously variable transmission configured as explained above, the rotation of the input shaft 15 is transmitted to the input disc 2 via the pressuring device 101. Then, the rotation of the input disc 2 is transmitted to the output disc 4 via the pair of the power rollers 8, 8, and the rotation of the output disc 4 is taken out through the output gear 18. When the torque is transmitted as set forth above, the input shaft 15 is pulled leftward in FIG. 3 as the pressuring device 101 operates, causing a leftward thrust load in FIG. 3 to be applied to the first ball-and-roller bearing 39. Further, the output gear 18 is pressed rightward in FIG. 3 via the input disc 2, the power rollers 8, 8, and the output disc 4 as the pressing device 101 is operated, causing a rightward thrust load in FIG. 3 to be applied to the second ball-and-roller bearing 40.
To change the rotational speed ratio of the input shaft 15 to the output gear 18, a pair of the drive pistons 35, 35 are displaced in the opposite directions from each other. As these drive pistons are displaced, the pair of trunnions 6, 6 are accordingly displaced in the opposite directions from each other. As a result, for example, the lower power roller 8 shown in FIG. 4 is displaced to the right in the drawing, while the upper power roller 8 shown in the drawing is displaced to the left in the drawing. This changes the direction of the force in the tangential direction, which force acts on the peripheral surfaces 8a, 8a of the power rollers 8, 8, and the inner surfaces 2a and 4a of the input disc 2 and the output disc 4, respectively. The change in the direction of the force causes the trunnions 6, 6 to rock in the opposite directions from each other in FIG. 3 around the pivots 5, 5 pivotally supported by the support plates 20, 20. As a result, as illustrated in FIGS. 1 and 2, the positions where the peripheral surfaces 8a, 8a of the power rollers 8, 8 are held against the inner surfaces 2a and 4a change accordingly, and the rotational speed ratio of the input shaft 15 to the output gear 18 changes.
As the component parts resiliently deform at the time of power transmission and the power rollers 8, 8 are displaced in the axial direction of the input shaft 15, the displacement shafts 7, 7 pivotally supporting the power rollers 8, 8 slightly move circularly about the supporting shafts 21, 21. This causes the outer surfaces of the outer rings 30, 30 of the thrust ball bearings 26, 26 and the inner surfaces of the trunnions 6, 6 to be relatively displaced. The relative displacement requires a small force because of the presence of the thrust needle bearings 27, 27 between the outer surfaces and the inner surfaces. Hence, only a small force is required to change the tilt angles of the displacement shafts 7, 7.
In the case of the conventional toroidal type continuously variable transmission which is constructed and which operates as set forth above, when transmitting a large torque, the torque or resistance required to rotate the first and second ball-and-roller bearings 39 and 40 inevitably increases, and the loss at the first and second ball-and-roller bearings 39 and 40 accordingly increases, thus failing to ensure satisfactory transmission efficiency in the entire toroidal type continuously variable transmission. This means that the torque required to rotate the ball-and-roller bearings increases as the load applied to the ball-and-roller bearings increases. In the case of the conventional toroidal type continuously variable transmission, the load based on the torque to be transmitted is directly applied to the first and second ball-and-roller bearings 39 and 40, so that the torque required for rotating the two ball-and-roller bearings 39 and 40 increases with resultant deteriorated transmission efficiency as described above.
A toroidal type continuously variable transmission which has been studied mainly as an automotive transmission is equipped with a toroidal speed changing mechanism comprised of an input disc and an output disc which have arc-shaped recessed sections as the surfaces facing each other, and a rotatable power roller held between the two discs. The input disc is driven and coupled to a torque input shaft such that it is able to move in the axial direction of the torque input shaft, whereas the output disc is installed to face against the input disc such that it is able to relatively rotate with respect to the torque input shaft and the movement thereof away from the input disc is limited.
In the toroidal speed changing mechanism constructed as set forth above, the rotation of the input disc causes the output disc to rotate in the opposite direction via the power roller, so that the rotational movement input to the torque input shaft is transmitted to the output disc as the rotational movement in the opposite direction and taken out through an output gear which rotates integrally with the output disc. At this time, the speed is increased from the torque input shaft to the output gear by changing the tilt angles of the rotating shafts of the power rollers such that the peripheral surfaces of the power rollers abut a portion near the outer periphery of the input disc and a portion near the center of the output disc, respectively. Conversely, the speed is decreased from the torque input shaft to the output gear by changing the tilt angles of the rotating shafts of the power rollers such that the peripheral surfaces of the power rollers abut a portion near the center of the input disc and a portion near the outer periphery of the output disc, respectively. A speed change ratio in midway between the two can also be continuously obtained by properly adjusting the tilt angles of the rotating shafts of the power rollers.
A loading cam device for increasing or decreasing the pressing force toward the input shaft in accordance with the input torque is disposed between a loading nut secured to an end of the torque input shaft, the end facing the input disc, and the input disc so as to adjust the frictional force generated between the input disc and the power rollers and between the power rollers and the output disc to proper values at all times. The loading cam device is constituted by: an input disc which has a cam surface extending in the circumferential direction and having projections and depressions, and which is engaged with the torque input shaft to rotate integrally with the torque input shaft; an input disc which has a similar cam surface provided such that it faces the cam surface of the cam disc and which rotates relatively to the torque input shaft; a nearly annular holder placed between the two discs; and rolling members provided at a plurality of openings formed in the circumferential direction of the holder. Each of the rolling members is held so that it is free to roll, the radial direction of the holder being the central axis, at an opening of the holder and it is installed so that the side surfaces thereof abut against the projections and depressions of both cam surfaces of the cam disc 102 and the input disc.
The torque input shaft is supported by a bearing provided on the end facing the input disc and an input bearing provided on the end facing the output disc such that it is rotatable in relation to the casing of the toroidal type continuously variable transmission. The output gear is also supported rotatably in relation to the casing of the toroidal type continuously variable transmission by an output bearing provided on the rear surface of the output gear. The output bearing and the input bearing are respectively held with their rear surfaces butted against each other by a support member joined to the casing of the toroidal type continuously variable transmission; if angular bearings are used, then they are combined so that the directions of the contact angles thereof are opposite from each other.
To transmit the rotational torque supplied to the torque input shaft to the output gear, the loading cam device is operated to move the torque input shaft toward the input disc and to move the output gear toward the output disc. This causes the input bearing to be subjected to a thrust load directed toward the input disc, and the output bearing to be subjected to a thrust load directed toward the output disc.
In the single-cavity, toroidal type continuously variable transmission equipped with only one toroidal speed changing mechanism as explained above, the reaction forces of the input disc and the output disc are large and the thrust loads applied to the output bearing and the input bearing are high, resulting in markedly increased rotational resistance of the bearings in such a case where a high torque is transmitted. This has been posing a problem in that a great loss in the dynamic torque is inevitable, making it impossible for the transmission to maintain satisfactorily high transmission efficiency.